1. Field of the Invention
The present invention relates to an apparatus for and a method of drilling offset wells for producing hydrocarbons.
2. General Background
Conventional wells drilled for producing hydrocarbons comprise a single vertical bore hole which pierces a hydrocarbon-containing formation. The effective surface area of the well is equal to the perimeter of the bore hole times the thickness of the formation. If the hydrocarbons are relatively fluid and the flow rate of the hydrocarbons is sufficiently fast, this surface area is satisfactory. When the hydrocarbons are relatively viscous, or for some other reason the flow rate is relatively slow, it is desirable to maximize the effective surface area of the well. Some oil producers increase the effective surface area of conventional vertical wells by drilling horizontal wellbores into the formation from the vertical bore hole. Various methods exist for producing these horizontal wellbores.
Four different technologies are currently commercially available for drilling horizontal wellbores. The primary distinction between the technologies is based on the rate of change in the inclination angle incorporated in the transition from vertical to horizontal:
(1) instantaneous, in the shaft-radial method, PA1 (2) short (11/4.degree.-3.degree. per foot), PA1 (3) medium (18.degree.-30.degree. per 100 feet), PA1 (4) long, or conventional (11/2.degree.-6.degree. per 100 feet).
The short-radial technique, often referred to as the "wagon wheel" method because of the resemblance when viewed from above, involves the drilling of a large diameter shaft vertically to the oil reservoir. Drilling equipment is lowered into the shaft, and horizontal wells are drilled radially along the perimeter of the shaft. Although a vast amount of reservoir contact results from this technique, the initial expense and considerable construction time, in addition to depth limitations and safety considerations associated with the shaft, limit the practical applicability of this method to most oil reservoirs.
The short radius, or drain hole, method involves drilling several holes radially from an existing wellbore. The transition from vertical to horizontal is performed in from 1 foot to 30 feet, depending on whether the knuckle joint (wigglies) or coiled tubing method of drainhole drilling is employed. This method maximizes the amount of horizontal bore length generated for the total hole drilled and allows the use of the existing casing in a wellbore. Its popularity has been severely restricted due to the lack of existing logging techniques and completion technologies suitable for use with the short radius approach. This has resulted in most of the drainhole completions to date being left uncased, or barefoot.
The newest of the horizontal well drilling technology is that associated with medium radius drilling. The 350 foot radius of curvature build section allows for a reasonably short transition from vertical to horizontal, yet facilitates longer drainhole lengths than the short radius wellbore. The major drawback of this system, as with the short radius approach, is that compatible logging and completion equipment has not yet been developed.
The long radius, or conventional, method has been the most popular in the last few years, among the major oil companies, particularly in the case of using horizontal drilling technology in conjunction with a new well. Although several thousand feet of drilling is required to make the transition from vertical to horizontal, this approach has the distinct advantage of being compatible with much of the existing logging equipment and completion methods.
Mining techniques have been used for oil production in numerous countries over many centuries. Only a few projects have been undertaken in the last century, however.
There has been a resurgence of interest in oil mining both by governments and the private sector in recent years. The reason for this interest is the high percentage of recovery of oil in place afforded by this technology. Present estimates are that 300 billion barrels of light crude and 200 billion barrels of heavy oil will remain in place in the U.S. and Canada after development by existing primary and secondary recovery methods.
Primary recovery refers to the development of reserves by conventional surface wells and pumping equipment; nothing is added to the reservoir to increase or maintain drive energy nor to sweep the oil towards the well. Primary recovery methods tend to produce 15-20 percent of the oil in place.
Secondary recovery involves the addition of fluid to the reservoir to supplement depleted reservoir energy pressure and sweep the oil towards and into the production well. Waterfloods and steamfloods are typically secondary recovery processes. Secondary methods normally recover 15-20 percent of the oil in place.
U.S. government studies estimate that 50% to 90% of the "lost" oil could be recovered using oil mining techniques.
The oil mining industry can be classified into two categories depending on whether the primary operations are located 1) on the surface or 2) underground. Surface mining has proven over the years to be, by far, the cheapest and simplest extraction method. Basically, the overburden material is stripped away to expose the oil bearing host rock. This host rock is then directly mined. Both the overburden stripping and host rock mining operation require extensive use of heavy earth moving equipment, such as draglines, bulldozers, and bucketwheel excavators. Either large dumptrucks or conveyor systems are then employed to transport the oil material to a processing facility.
Variations of surface mining methods include (1) terrace pit, (2) strip mining, and (3) open pit mining. The primary limitation of all surface mining methods is the depth of overburden material that can be practically handled (generally 250 feet or less). Advantages of surface methods include low extraction cost and high percentage of recovery of the oil in place.
The second mining group include systems that occur underground. Although generally more expensive than surface systems, underground methods hold far more potential for oil recovery due to the far greater depths at which the systems are feasible. Underground systems can be divided into (1) processes in which the oil bearing rock is physically removed from the mine, as in direct stoping and block caving systems, and (2) processes in which mining is only a means to gain access to the proximity of the oil bearing formation, in order to limit the amount of drilling that must be done to produce the reservoir. Underground drainage methods include (1) shatter and drain systems, (2) drainage with steam methods, and (3) gravity drainage.
The main advantage of the underground drainage methods is in allowing the spacing of wells in a much denser configuration than would be possible if wells were drilled from the surface. This results in lower recovery cost per barrel of oil and a higher recovery percentage of oil in place as compared to conventional surface wells. Limitations or requirements of potential underground mining reservoir candidates include:
(1) location of a competent rock layer adjacent the interval to be produced;
(2) formation temperature not exceeding worker comfort levels (although this limit changes by geographical location in the U.S., the depth limitation due to temperature levels is generally in the range of 4,000-6,000 feet).
Underground mining projects normally involve high initial capital costs, often in the range of 20 million to 350 million dollars. This would somewhat limit the investment sources to governments and sizable private entities.
In the early 1980's, Petroleum Mining Corp. of Dallas, Tex. considered an oil mining operation involving digging 10 foot high by 10 foot wide tunnels underneath a formation, then drilling a plurality of drain holes up into the formation in a number of different orientations from a number of drill rooms positioned adjacent the tunnels. Construction of these tunnels would be very expensive, and there would always be the danger that the tunnels might collapse on the underground workers.
U.S. Pat. No. 4,519,463 discloses a method of producing hydrocarbons comprising drilling a primary wellbore having a curved section drilled at an angular rate of build of from about 2.5.degree. to 6.degree. per 100 feet of primary wellbore length and an essentially horizontal section at the end of the curved section. Drainhole wellbores are then drilled from the essentially horizontal section, the drain hole well bores including a curved portion drilled at an angular rate of build up from about 0.2.degree. to about 3.degree. per foot of drain hole wellbore length, and a substantially straight portion at an angle of approximately 90.degree. from the longitudinal axis of the essentially horizontal section of the primary wellbore. The drainhole wellbores, due to the angular rate of build of their curved sections, cannot be logged or completed with existing logging and completion equipment.